1. Field of the Invention
This invention relates to a sintered aluminous abrasive grain and a method of producing the same used for grinding wheel, coated abrasive, lapping composition used for lapping and so forth.
2. Prior Arts
Conventionally there have been provided a method for producing a sintered aluminous abrasive grain wherein alumina containing materials such as bayer alumina powder, bauxite powder and so forth are formed into particles with a binder and then sintered at a high temperature of 1600.degree. C. or higher. Another method has been also provided wherein alumina monohydrate used as a raw material is treated by a sol-gel process and sintered at a low temperature of 1500.degree. C. or lower. The former method produces abrasive grains including alpha alumina of a large crystal size such as 3-10 microns and consequently the application thereof is limited to heavy duty grinding. The latter method produces abrasive grains having a crystal size of alpha alumina below 2 microns.
Japanese patent application laid open under No. 56-32369 whose related U.S. patent applications Ser. No. 53,347 and Ser. No. 145,383 are patented under U.S. Pat. No. 4,314,827 discloses a sintered alumina abrasive grain produced from a substantially calcium ion- and alkali metal ion-free alumina monohydrate by a sol-gel process. In this technology, at least one modifying component is added into a colloidal dispersion of alumina. The modifying component is added in a form of salt. Proposed for the modifying component is at least 10 vol % of zilconia and/or hafnia and at least 1 vol % of spinel derived from alumina and at least one oxide of a metal selected from cobalt, nickel, zinc, or magnesium.
Japanese patent application laid open under No. 57-20762 which is related to U.S. patent applications Ser. No. 267,495 and 330,123 discloses a sintered abrasive grain from a sol-gel process which is characterized in that sodium and calcium are contained in the grain.
Japanese patent application laid open under No. 60-231462 whose related U.S. patent application Ser. No. 572,106 is patented under U.S. Pat. No. 4,623,364 discloses a sintered alumina abrasive grain from a sol-gel process wherein seed crystal of alpha alumina is used. Namely, alpha alumina particles of submicron size are mixed as a nucleator into a dispersion of hydrate alumina particles in order to lower a transition temperature of a gel alumina from gamma form to alpha form. Also disclosed is that source compounds for Si, Cr, Mg and Zr may be added to a gel as grain growth inhibitors. In short, dense alpha alumina crystal particles are obtained by adding alpha alumina particles of submicron size as a nucleator and thereby permitting the manufacture by low temperature sintering.
Japanese patent application laid open under No. 63-97681 discloses a sintered alumina abrasive grain from a sol-gel process obtained by vacuum degassing of a sol. It is required that occluded air in a suspension shall be removed and further aggregation particles shall be removed from the suspension. It is described that the addition of particle growth inhibitors such as Mg, Ca, Co, Si etc. shows no effect. In short this invention provides dense abrasive grains by removing occluded air in the suspension and removing the aggregation particles from the suspension and thereby forming alpha alumina nuclei at 1000.degree. C.
The modifying component added in the form of salt in the above mentioned Japanese patent application laid open under No. 56-32369 has an effect to accelerate gelation, as a result the addition into the colloidal dispersion causes rapid gelation and results in remarkable increase of viscosity. Consequently uniform mixture of the modifying component is difficult and uniform dispersion of composition can not be attained. As a result nonuniform portion is produced, which results in low grinding efficiency. Sintering temperature for abrasive grains is at least 1250.degree. C. but not above the melting point. Accordingly, in order to obtain a superior grinding efficiency, it is inevitable that alumina component must be of complete alpha alumina crystal.
Japanese patent application laid open under No. 57-20762 discloses that the abrasive grains obtained from a sol-gel process have superior efficiency than conventional fused alumina abrasive grains even if the grains contain sodium which is usually contained as an impurity in alumina raw materials and calcium which is contained in water as an impurity. Namely, the effect of intentional addition of alkaline metal elements such as sodium and calcium is not recognized.
In accordance with the method of Japanese patent application laid open under No. 60-231462, huge crystal particles are present in addition to the crystal particles of submicron size. It is construed that said huge crystal particles are caused from nucleator, because nucleator inevatably includes large alpha alumina particles in addition to alpha alumina particles of submicron size even if most careful attention is paid. Further smallest particles of alpha alumina particles of submicron size are larger than alpha alumina crystal nuclei formed in a gel at about 1000.degree. C., so that nonuniform growth of crystal particles is caused.
In the method disclosed in the Japanese patent application laid open under No. 63-97681, the nucleation of alpha alumina takes place at random so that the crystal of alpha alumina becomes nonuniform. This invention also intends to obtain a complete alpha alumina crystal at a low temperature by lowering the transition temperature of alumina to alpha alumina crystal.